The present invention relates to a method and an apparatus for triggering a machine part, which can be selectively activated by a trigger signal in order process a workpiece with a high positional accuracy. Such a method and such an apparatus are particularly required for the production of liquid crystal displays (LCDs) or solar cells. However, the present invention is not restricted to these exemplary embodiments, although they are preferred applications.
One known method for position-accurate triggering of a movable machine part is described in DE 101 49 828 A1. In this case, the machine part is an engraving stylus for engraving a printing cylinder. The engraving stylus is moved relative to the printing cylinder along an axis of movement which runs parallel to the longitudinal axis of the printing cylinder. The drive for the engraving stylus is a stepping motor which produces a defined forward movement of the engraving stylus along the axis of movement in response to each pulse of a pulse train. A present axial position of the engraving stylus can therefore be determined by counting the pulses. In order to ensure high positioning accuracy of the engraving stylus, DE 101 49 828 A1 proposes a method for correcting position discrepancies. This method includes that axial measurement positions are defined on the forward movement path of the engraving stylus before the engraving process, and a measurement apparatus is used in order to determine position discrepancies at the measurement positions. The position discrepancies are provided as correction values in a correction value memory. During the engraving process, the position discrepancies of the engraving stylus are corrected by means of the values from the correction value memory.
The known method requires the correction values to be separately recorded. The position accuracy on triggering of the machine part depends on the density of the correction values. It may be disadvantageous to record a plurality of correction values due to the burden associated with this. However, it is even more problematic that the rate of movement of the machine part relative to the workpiece is limited, in the case of the known method, by the speed at which the position correction can be carried out. Since the correction values must be read from a memory and must be processed in the known method, the known method either requires rapid and expensive control computers, or the machine part can be moved forward only rather slowly. The known method is therefore not well suited for the initially mentioned preferred applications.